The ground screw is just 'touching' the box? That doesn't sound like a good idea at all. That probably means an intermittent or high resistance connection at best.
Do yourself(and your gear) a favor: shut off the breaker, yank the outlet, run a self tapping screw through the back of the box and connect a chunk of 12ga bare copper between it and the ground screw on your outlet. Next, go to Rat Shack and get yourself an a/c circuit tester as suggested by Gs5556 above. Test accordingly.
If all is well, you're ready to reconnect your power conditioners and see that they are working properly(one of them could actually be part of the problem.) If not, it would be a good idea to consult an electrician to rectify the problem.